Laser induced fluorescence diagnostic for velocity distribution functions:applications,physics,methods and developments

With more than 30 years of development,laser-induced fluorescence(LIF)is becoming an increasingly common diagnostic to measure ion and neutral velocity distribution functions in different fields of studies in plasma science including Hall thrusters,linear devices,plasma processing,and basic plasma physical processes.In this paper,technical methods used in the LIF diagnostic,including modulation,collection optics,and wavelength calibration techniques are reviewed in detail.A few basic physical processes along with applications and future development associated with the LIF diagnostics are also reviewed.

Quantitative measurement of hydroxyl radical(OH) concentration in premixed flat flame by combining laser-induced fluorescence and direct absorption spectroscopy

An accurate and reasonable technique combining direct absorption spectroscopy and laser-induced fluorescence（LIF）methods is developed to quantitatively measure the concentrations of hydroxyl in CH;/air flat laminar flame. In our approach, particular attention is paid to the linear laser-induced fluorescence and absorption processes, and experimental details as well. Through measuring the temperature, LIF signal distribution and integrated absorption, spatially absolute OH concentrations profiles are successfully resolved. These experimental results are then compared with the numerical simulation. It is proved that the good quality of the results implies that this method is suitable for calibrating the OH-PLIF measurement in a practical combustor.

Quantitative measurements of one-dimensional OH absolute concentration profiles in a methane/air flat flame by bi-directional laser-induced fluorescence

The one-dimensional （1D） spatial distributions of OH absolute concentration in methane/air laminar premixed flat flame under different equivalence ratios at atmospheric pressure are investigated by using bi-directional laser-induced flu- orescence （LIF） detection scheme combined with the direct absorption spectroscopy. The effective peak absorption cross section and the average temperature at a height of 2 mm above the burner are obtained by exciting absorption on the Q1（8） rotational line in the A2∑＋ （Dt = 0） ←- X2∏ （v = 0） at 309.240 nm. The measured values are 1.86×10-15 cm2 and 1719 K, respectively. Spatial filtering and frequency filtering methods of reducing noise are used to deal with the experi- mental data, and the smoothing effects are also compared using the two methods. The spatial distribution regularities of OH concentration are obtained with the equivalence ratios ranging from 0.8 to 1.3. The spatial resolution of the measured result is 84μm. Finally, a comparison is made between the experimental result of this paper and other relevant study results.

Simultaneous laser-induced fluorescence and contactless-conductivity detection for microfluidic chip

A combined detection system involving simultaneous LIF and contacfless-conductometric measurements at the same place of the microfluidic chip was described. The LIF measurement was designed according to the confocal principle and a moveable contactless-conduetivity detector was used in C^4D. Both measurements were mutually independent and advantageous in analyses of mixtures. Various experimental parameters affecting the response were examined and optimized. The performances were demonstrated by simultaneous detection of Rhodamine B. And the results showed that the combined detection system could be used sensitively and reliably.

Numerical analysis of quantitative measurement of hydroxyl radical concentration using laser-induced fluorescence in flame

The aim of the present work is to quantitatively measure the hydroxyl radical concentration by using LIF（laserinduced fluorescence） in flame.The detailed physical models of spectral absorption lineshape broadening,collisional transition and quenching at elevated pressure are built.The fine energy level structure of the OH molecule is illustrated to understand the process with laser-induced fluorescence emission and others in the case without radiation,which include collisional quenching,rotational energy transfer（RET）,and vibrational energy transfer（VET）.Based on these,some numerical results are achieved by simulations in order to evaluate the fluorescence yield at elevated pressure.These results are useful for understanding the real physical processes in OH-LIF technique and finding a way to calibrate the signal for quantitative measurement of OH concentration in a practical combustor.

Absolute density measurement of nitrogen dioxide with cavity-enhanced laser-induced fluorescence

The absolute number density of nitrogen dioxide（NO2） seeded in argon is measured with cavity-enhanced laserinduced fluorescence（CELIF） through using a pulsed laser beam for the first time. The cavity ring down（CRD） signal is acquired simultaneously and used for normalizing the LIF signal and determining the relationship between the measured CELIF signal and the NO2 number density. The minimum detectable NO2 density down to（3.6±0.1）10^8 cm^-3 is measured in 60 s of acquisition time by the CELIF method. The minimum absorption coefficient is measured to be（2.0±0.1）10^-9 cm^-1, corresponding to a noise equivalent absorption sensitivity of（2.2±0.1）10^9 cm.^-1Hz^-1/2. The experimental system demonstrated here can be further improved in its sensitivity and used for environmental monitoring of outdoor NO2 pollution.

Ion Transport Measurements in a Multi-Dipole Argon Plasma by Broadband Laser Induced Fluorescence

Argon ion laser induced fluorescence measurements were carried out in a multipolar filament discharge with a broadband diode laser centered on 668 nm, which stimulated a transition from the metastable state in Ar（Ⅲ） 3d4F7/2 to 4p4D0 5/2. The intensity of the induced fluorescence at 442 nm was maximized by the optimization of the discharge parameters and the laser power. From the recovery of the background fluorescence after the laser was turned off, the ion diffusion coefficient was deduced and compared with the result inferred from the experiments of ion acoustic wave （IAW） damping.

Detection of Petroleum Pollutants by Using Laser Induced Fluorescence Imaging

A new laser induced fluorescence imaging system used for detecting and locating the petroleum pollutants is presented. A unique feature of the system was using a combination of a frequency tripled Nd:YAG pulse laser and an image intensified CCD camera as imaging detector. The delay time of the system and the exposure time of the CCD are controlled by a delay generator. Petroleum pollutants on various backgrounds had been detected and located by this system. This imaging technique provides a useful noninvasive approach for the detection of petroleum pollutants, which can be used in remote sensing.

Hyperspectral imaging and remote trace detection of cis-1,3,4,6-tetranitrooctahydroimidazo-[4,5 d]imidazole(BCHMX)compared with traditional explosives using laser induced fluorescence

cis-1,3,4,6-Tetranitrooctahydroimidazo-[4,5 d]imidazole(BCHMX)is an advanced energetic compound that expected to spread worldwide in the near future.Since,no approved remote detection methods were reported in current literature for this material,we performed hyper-spectral imaging and laser induced fluorescence(LIF)to a BCHMX sample under low laser fluence for determining the optimum laser wavelength used in any future BCHMX-LIF based remote detection systems.For this purpose,an experimental setup consisted of a sun spectrum lamp and hyper-spectral camera was built to illuminate and image white powder samples of BCHMX in comparison with the traditional explosives,HMX(1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane),RDX(1,3,5-trinitro-1,3,5-triazacyclohexane),PETN(2,2-Bis[(nitroxy)methyl]propane-1,3-diyldinitrate).The imaging reveals strong BCHMX sample absorption contrast among other samples at wavelength ranging from 400 to 410 nm.When light source was replaced by a 405 nm laser diode illuminator,a strong BCHMX sample LIF at the spectral range from 425 to 700 nm was observed under low laser fluence condition of 0.1 mJ/cm^(2).Finally,we demonstrated successfully the ability of the 405 nm LIF and the hyperspectral imaging technique to detect finger print traces of BCHMX on white cellulose fabric from a distance of 15 m and a detection limit of 1 mg/cm^(2).

Laser Induced Fluorescence Spectroscopy of IrN

High resolution laser induced fluorescence, spectra of IrN in the spectral region between 394 and 520 nm were recorded using laser vaporization/reaction free jet expansion and laser induced fluorescence spectroscopy. Seven new vibronic transition bands were observed and analyzed. Two Ω=1 and five Ω=0 new states were identified. Least squares fit of rotationally resolved transition lines yielded accurate molecular constants for the upper states. Spectra of isotopic molecules were observed, which provided confirmation for the vibrational assignment. Comparison of the observed electronic states of IrB, IrC, and IrN provides a good understanding of the chemical bonding of this group of molecules.

Research of Confocal Laser Induced Fluorescence Detection System for Micro-fluidic Chip

The characteristics such as signal noise ratio（SNR） and sensitivity of the fluorescence detection system for micro-fluidic chip influence the performance of the whole system extremely. The confocal laser induced fluorescence detection system is presented. Based on the debugging of optical and circuit modules, the results of detecting the samples are given and analyzed theoretically, and the improved project is put forward.

Production of Polyclonal Antibody of Morphine and Determination of Morphine in Urine by Capillary Electrophoresis Immunoassay with Laser-induced Fluorescence Detection

N-Conjugated antigen was synthesized and polyclonal antibody with high specificity was obtained from immunizing animals. With this polyclonal antibody, a rapid and efficient CEIA-LIF method was developed to determine the free morphine in urine of abusers. The detection limit was calculated to be 40 ng/mL. Simulated urine samples were analyzed with good recoveries, which showed the feasibility of its application in specific morphine determination in urine of morphine abusers.

Determination of Amino Acids in an Individual Erythrocyteby Capillary Electrophoresis with Intracellular FITC-derivatization and Laser-induced Fluorescence Detection

A novel approach for analysis of amino acids in individual erythrocytes was established. In this method, the derivatization reagent was introduced into the living cells by electroporation. After derivatization, the amino acids in a single cell were determined by capillary electrophoresis with laser-induced fluorescence detection.

Mixed Micellar Electrokinetic Chromatographic Analysis of Colistin, Polypeptide Antibiotic, Using Laser-Induced Fluorescence Detection

The main goal of this work was to quantify the detection of colistin at low levels in urine samples through the practical application of mixed surfactant micellar electrokinetic chromatography–laser-induced fluorescence (MEKC-LIF) analysis method using its advantage of sensitivity and to examine direct injection of biological samples. Colistin (po- lymyxin E) has neither strong UV chromophore nor fluorophore. So, its assay for metabolism, pharmacokinetics studies for bioavailability and bioequivalence are difficult because of poor detectability. Therefore an enhanced UV or fluores-cence detection by chemical derivatization is required. MEKC-LIF method was proposed for colistin with a 488/520 nm argon-ion laser using a pre-CE derivatization with fluorescein isothiocyanate (FITC). Borate buffer was used as background buffer (BGB). The different parameters affecting the proposed derivatization reaction including concentration of the derivatizing reagent, reaction time and temperature were studied and optimized. The derivative was stable for up to 3 days. Different micelles (TX-100 and SDS) were examined as BGB additives separately but negative-charged mixed micelles (SDS/TX-100) were shown to be the best additive to BGB for the analysis of colistin particularly in human urine as they enhance both selectivity and sensitivity of the proposed method. BGB was used with pH 9.5, 10 kV, 8 s inj time, capillary length 75 cm × 75 μm ID (66 cm effective length), detection was LIF Ex 488 nm;Em 520 nm. The method was applied to colistin analysis in human urine and the recovery was > 98% (n = 5). LOD and LOQ in urine after pre-column derivatization using FITC were 100 and 250 ng/ml, respectively. Urine samples were analysed by direct injection without sample pre-treatment. The mechanism of enhancement of fluorescence of the derivative by surfactant was proposed.

基于LIF对水包油型轻质油乳化液的分类识别

海面溢油是海洋污染的重要形式之一,溢油在风化迁移过程中,形成如水包油,油包水,水包油包水等乳化液,其中水包油类型的乳化液受水分子的影响较大,荧光特性不突出,导致轻质油乳化液分类的困难,因此如何进行高效识别,对污染治理有重要意义。选取常见的几种轻质油分别与海水、乳化剂按照不同配比混合配置水包油类型的轻质油乳化液。采用实验室搭建的便捷激光诱导荧光(LIF)系统探测轻质油乳化液的荧光光谱。构建麻雀搜索算法(SSA)优化支持向量机(SVM)的分类模型(以下简称SSA-SVM),实现乳化阶段溢油的分类识别。采用主成分分析法(PCA)对荧光光谱进行降维,选取累计贡献率为99%的前三个主成分作为输入,轻质油的种类作为输出;采用SSA迭代得到SVM的最优参数,并构建SSA-SVM分类模型;最后将测试集样本代入到模型中进行分类识别,识别准确率为100%。该研究同时构建了粒子群算法(PSO)优化支持向量机模型(简称PSO-SVM)和遗传算法(GA)优化支持向量机模型(简称GA-SVM)作为对照,实验结果表明,SSA-SVM相比PSO-SVM和GA-SVM,测试集的轻质油乳化液分类识别准确率同比提升1.77%和3.04%;并且适应度曲线在第2代就达到最高,优于PSO的第4代和GA的第36代,收敛速度更快。采用激光诱导荧光技术实现了水包油类型的轻质油乳化液的分类识别,推进了海面溢油区域分类探测机理的发展;提出的SSA-SVM模型,为轻质油乳化液的分类识别研究提供了新思路。

超燃冲压发动机羟基/煤油-PLIF同步测量研究

应用一套500 Hz PLIF(Planar laser induced fluorescence)系统开展了煤油燃料超燃冲压发动机实验研究,实现了超燃冲压发动机燃烧室内火焰结构和煤油分布可视化测量。通过对OH-PLIF和煤油-PLIF同步测量方法分析,揭示了超燃冲压发动机煤油掺混燃烧振荡特性规律。利用PLIF图像的几何特征、强度特征和动态模态分解,分析了煤油掺混过程对火焰传播及燃烧特性的影响。研究表明,在煤油单独燃烧阶段,煤油主要分布在燃烧室上游。OH基受燃料分布的影响,在燃烧室内重复性地聚集和扩散,呈现破裂带状。动态模态分解结果表明,煤油掺混过程对燃烧振荡存在影响。

基于双色PLIF的推进剂燃烧场定量测温方法

在燃烧学研究领域中,平面激光诱导荧光(PLIF)可以有效鉴别组分类型,获取火焰中微量组分的二维平面成像以及通过分子振动能级的分布测量温度。本文基于双色PLIF技术搭建PLIF测温实验系统,以推进剂燃烧作为研究对象,采用—OH基中的Q_(2)(11)与P_(1)(7)2条谱线开展双色PLIF技术测温实验,结合采集到的—OH基荧光图像,获取推进剂火焰二维温度场分布。通过火焰二维温度场分布情况与热电偶的测温结果进行对比及误差分析。对比结果表明:双色PLIF测温技术的测温误差为12.2 K,不确定度小于5%。该方法具有非接触、高灵敏度等特性,适用于稳态燃烧场温度测量。

基于PLIF检测的生物质挥发分燃烧多环芳烃生成特性

针对生物质挥发分燃烧过程,采用平面激光诱导荧光技术对火焰中1~2环、3~5环多环芳烃(PAHs)的二维分布进行了研究,分析对比了升温速率(1 000 K/min和1 500 K/min)、O_(2)体积分数(21%~50%)等关键因素对木屑、核桃壳颗粒挥发分燃烧过程中PAHs生成与转化特性的影响.结果表明:提高升温速率,生物质挥发分燃烧过程中1~2环和3~5环PAHs信号强度峰值显著增加,同时升温速率提高促进了1~2环和3~5环PAHs向小分子碳氢化合物转化;1~2环和3~5环PAHs信号强度峰值随O_(2)体积分数提高而增加,这是由于增加的O_(2)体积分数促进了PAHs的释放.但是由于O_(2)体积分数的提高同样增加了OH自由基浓度,导致PAHs消耗加快,使得1~2环和3~5环PAHs信号强度持续时间随O_(2)体积分数提高先增加后减少.木屑和核桃壳颗粒挥发分燃烧过程中生成的3~5环PAHs信号强度峰值和信号强度持续时间较为一致,但木屑颗粒生成的1~2环PAHs信号强度峰值更高,这可能是由于木屑颗粒脂肪烃和芳香环含量更高,同时燃烧过程中生成的OH自由基较多,有利于多环PAHs向1~2环PAHs的转化.

面向双级旋流燃烧器的10kHz滤波瑞利散射测温和CH_(2)OPLIF测量研究

本文开展了常压下双级径向分层旋流燃烧器内旋流火焰的二维温度场及甲醛浓度场测量研究,发展了10kHz测量频率的滤波瑞利散射测温技术.首先,借助Hencken平面火焰炉构建了滤波瑞利散射信号与温度的校准关系.基于校准后的滤波瑞利散射系统,获得了旋流火焰的二维温度场数据:实验分析主要聚焦于甲烷-空气混合物在不同当量比(0.65至1.05)下的温度场变化.在当量比为0.85的单旋流火焰中,观察到火焰形状由V形向M形的转变,这归因于热扩散效应与空气热传导效应的相互作用.此外,在双旋流火焰中,研究发现相同运行条件下,值班火焰与主火焰在燃烧过程中发生从融合到分层的变化.本文详细描述了双旋流火焰分层现象期间的相平均温度场分布,并与瞬时CH_(2)O分布进行了对比.

大气湍流对LIF探测海面溢油影响的BRRDF仿真研究

及时定位海面溢油事故发生地并监测海面溢油信息对海洋污染治理和海洋生态环境保护有重要意义。激光诱导荧光(LIF)可搭载于无人机构建LIF雷达系统并应用于海洋环境监测。LIF雷达系统以大气为传输信道,大气湍流会导致信号衰落。双向反射再辐射分布函数(BRRDF)利用受激辐射荧光光子的权重、位置和方向描述物质的荧光特性,为LIF探测海面溢油提供理论指导。基于大气湍流理论和蒙特卡罗方法模拟海面溢油的受激辐射荧光过程,建立光子传输模型进行海面溢油BRRDF的仿真研究。仿真分析不同湍流强度下,光强闪烁、光束漂移、光束扩展效应对海面溢油BRRDF的影响。仿真结果表明,海面溢油受激辐射荧光信号具有各向同性,其强度与激发光的强度成正比。光束漂移效应对荧光漂移范围影响微弱,可以忽略;在弱大气湍流下(湍流强度参数σ_(R)^(2)<1),BRRDF所表征的荧光信号强度集中在2×10^(-5)~5×10^(-5)范围内,荧光光斑半径由1mm扩展至5mm,LIF雷达系统可以正常进行探测工作;大气湍流强度到达中等后(σ_(R)^(2)≈1),荧光信号强度在10^(-5)~10^(-4)范围内波动,荧光光斑半径扩展至15mm,此时大气湍流的不利影响无法忽视,LIF雷达系统探测到的荧光信号不够稳定,最终导致光谱不理想。在实际探测中,可通过适当提高探测器性能参数来增强系统的抗湍流能力。强大气湍流下(σ_(R)^(2)=25),出现光斑破裂现象,荧光信号分散并衰落至某一阈值之下,LIF雷达系统无法探测到荧光信号。文中的分析与提出的可行的研究建议,为LIF探测海面溢油相关研究提供参考依据,对LIF雷达系统设计具有一定的参考价值。